The present research addresses the hypothesis that lamina I spinothalamic tract (lamI-STT) neurons are an essential substrate for the central representation of nociception and thermoreception as specific sensory modalities, i.e. that they constitute a central pathway that contains the "labeled lines" of specificity theory for pain and temperature sensation. These experiments focus on the functionally selective organization of this pathway and particularly concentrate on examining attributes which may reflect the distinct functional and anatomical classes of nociceptive and thermoreceptive laml-STT neurons identified physiologically in previous work in the cat. The new lectin (PHA-L) anterograde tracing method will be used to precisely determine the multiple laml-STT a thalamic termination sites, to characterize their morphology and to identify their pathway. The chemical and anatomical selectivity of these terminal projections will be tested with immunohistochemical the detection of endogenous neurochemical, and double-labeling methods for multiple retrograde tracers for the identification of segregated afferent cell groups. In addition, physiological experiments will complement the anatomical localization of the ascending lamI-STI spinal pathway with the individual localization of the axons of identified cells by antidromic activation from a microelectrode at L2. These experiments will directly determine whether lamI-STT axons project separately from the classical STI, as some have proposed, and whether nociceptive and thermoreceptive axons course together. The effects of tonic descending influences on the functional selectivity of these lamI-STT cells and also other non-STT lamina I cells will be examined with reversible spinal cold blocks. Lastly, the sensitivity of these cells to topically applied (intrathecal) morphine will be assessed in order to test their functional differentiation and, by comparing this sensitivity to reported characteristics of intrathecally produced analgesia, to verify the hypothesis that they play an essential role in nociception.